A transit-time ultrasonic flowmeter measures the flow velocity of fluids flowing in a conduit by determining the difference in transit time corresponding to ultrasonic signals transmitted upstream and downstream through a fluid flow path. Typically, the transit-time ultrasonic flowmeter includes a first ultrasonic transducer and a second ultrasonic transducer positioned along the fluid flow path. The first and second ultrasonic transducers may be inserted into the conduit (in-line configuration), or alternatively, they may be externally coupled to the outside of the conduit (clamp-on configuration).
An electrical excitation signal is applied to the first transducer which then transmits an ultrasonic signal. The ultrasonic signal travels through the fluid and the second ultrasonic transducer receives the ultrasonic signal and then generates an electrical measurement signal corresponding to the ultrasonic signal. The transit time in one direction is measured. The process is repeated in the other direction with the second ultrasonic transducer acting as a transmitter and the first ultrasonic transducer acting as receiver for measuring the transit time in the reverse direction. Finally, the difference in transit times in the two directions is used to determine the flow velocity of the fluid.
Ultrasonic transit-time flowmeters comprise hardware elements and software elements. Hardware elements, for example are the ultrasonic transducers and the control unit for transmitting, receiving and processing data derived from the ultrasonic signals. The most common implementation of ultrasonic transducers uses a piezoelectric crystal or piezoelectric ceramic. Various other implementations of ultrasonic transducers, such as magnetostrictive ultrasonic are also commonly known. Software elements are for example modules for controlling flow measurements, for generating input signals, for evaluating, converting and storing output signals, for processing signals and data etc.
When an ultrasonic transit-time flowmeter is installed as part of a “Safety Instrumented System” (SIS) it is required that the flowmeter also includes hardware and software measures to ensure the integrity of the flow measurement output, such that the system is able to detect and respond to any failures that may otherwise result in a false flow measurement value.
A number of possible failures in the flowmeter, if undetected, may result in a significant error in the flow measurement accuracy. Such a failure may have severe consequences if a simultaneous failure in a secondary device occurs. For example, the ultrasonic flow meter and a secondary device, for example, a level sensor, both fail, causing a gasoline storage tank to overflow.
Existing diagnostic measures may trap some of the failures by analyzing the actual fluid signal, but not to the level of confidence required for SIS applications.